Blow molding processes are commonly used to form hollow, thin-walled vessels such as bottles, drums, storage tanks, and other such containers that include a hollow inner chamber capable of containing and storing liquids or other substances. Generally, blow molding processes employ air pressure to expand a heated plastic tube known as a parison located inside a mold cavity. The inner surfaces of the mold cavity are generally designed to form the shape and contours of the outer surfaces of the vessel being molded. Air pressure is applied to expand the plastic parison until the parison conforms to the inner walls of the mold. The plastic cools and hardens against the mold cavity resulting in the desired vessel. Because vessels are typically designed to contain liquids or other substances, blow molded vessels commonly include an opening through which liquids or other substances can be poured into and out of the vessel. Such openings are often formed at the end of a necked portion of the vessel and provide access to the hollow chamber within the vessel. The necked portion can be arranged to accommodate a cap or other such component that can be removed or adjusted to gain access to the hollow chamber and replaced again to seal the opening of the vessel.
Examples of hollow thin-walled vessels formed by blow molding processes include bottles used for consumer products such as bottled water, liquid soap, shampoo, milk, and many other liquids. Because liquids can seep, run or otherwise move through small gaps or pathways caused by defects or flaws in sealing surfaces, it can be important that the sealing surfaces between the blow molded vessel and any cap or enclosing component be generally free from such defects and flaws that can cause gaps or pathways. If gaps and pathways through which liquids or vapors can travel (vapors can be stored in a vessel or can be generated, for example, by evaporation or vaporization of liquids contained within vessels) are present in the sealing surfaces between a vessel and its cap, the blow molded vessel may experience leaking issues that limit the effectiveness or usefulness of the vessel.
Blow molding processes are commonly used to form vessels because such processes offer the flexibility to make plastic, hollow, thin-walled vessels in a variety of shapes and sizes. Many different thermoplastic polymers can be used to blow mold vessels such as, for example, polyethylene, polypropylene, polyethylene terephtalate (also known as PET), and polyvinyl chloride (also known as PVC). Such thermoplastics in the form of small pellets or granules are typically heated until the thermoplastic is molten and further processed to form the parison.
Different techniques can be used to blow mold a vessel. Certain processes differ in how the parison is formed. For example “extrusion blow molding” employs an extruder with a rotating screw to push molten plastic through a die head to form the parison. The parison is extruded vertically so that gravity carries it down and positions the parison between two open mold halves positioned below the extruder. In one variation, the parison can be extruded around a blow pin that is already positioned within the open mold. The mold can be closed about the parison and blow pin to clamp the parison and blow pin into place. A gas, typically air, can be injected through the blow pin to expand the parison until the plastic forms the shape determined by the mold cavity. In another variation, the parison is extruded between the two open mold halves, the mold halves are closed about the parison, and the blow pin is pushed through an opening in the mold to engage with the parison. Air is injected into the parison through the blow pin, and the parison expands until it engages the inner surfaces of the mold to form the shape of the mold.
Extrusion blow molding tooling is relatively inexpensive and can be used for mass production of products. For example, extrusion blow molding can be used to manufacture large quantities of bottles for use in the sale of consumer goods. However, extrusion blow molding can produce vessels with dimensional variations, especially for inner surfaces of the vessel. This is to say that although extrusion blow molding is inexpensive and efficient, it may not be reliable enough if tight internal dimensional tolerances are required for a product. Therefore, when considering extrusion blow molding as a manufacturing process, the overall product design and the required tolerances need to be considered.
Another blow molding method is known as “injection blow molding.” In injection blow molding, a core pin is positioned within the cavity of a first mold. Molten plastic is injected around the core pin and into the cavity and cooled to form a solidified “preform.” The core pin and cavity are typically designed to form a preform that includes a neck portion on one end of the preform and a parison portion extending from the neck. The neck portion is more precisely formed because it is formed between the rigid surfaces of the mold cavity and the core pin. Likewise, the thickness of the walls of the parison portion are also more precisely formed between the rigid surfaces of the mold cavity and the core. The preform can be stored for later use or can be used immediately to form a vessel. When the preform is used to form a vessel, a blow molding process is used to form the parison into the body of the vessel. The preform can be positioned in a second mold with the neck portion clamped in the mold and the parison portion extending into the mold cavity. The mold and the neck portion can be arranged so that the neck portion is protected against any deformation due to pressure of elevated temperature of the mold. A blow pin can be positioned within the interior of the neck portion, and the parison portion of the preform can be heated. Air can be moved into the heated parison through the blow pin and the heated parison portion expands until the plastic forms a product based on the shape of the cavity of the mold. As compared to extrusion blow molding, injection blow molding can typically form more dimensionally precise and accurate components.
There is a need in the industry to develop innovative tooling and processes that form vessels that effectively seal difficult to seal products or combine the efficiency of extrusion blow molding with the dimensional precision of injection blow molding.